The cost of starting a business today is lower than ever. If you have an idea, pursue it. Or give one of these three arenas a hard look. I would (if I weren’t already tied up in social media).Recently I outlined the two most important rules to follow when deciding what market to attack for your new business. In turn, many readers asked me which markets are most ripe for starting new businesses.Here are the three areas I’d explore if I weren’t already knee deep in social media at Buddy Media:1.